Flexible circuits generally include a plurality of conductive traces that are supported on a base substrate such as a layer of flexible dielectric material. A key aspect of flexible circuits is that they offer attributes such as fine pitch traces, complex circuit designs and flexibility. Electronic packages, medical devices, hard disk drive suspensions and ink jet printer pens are common applications for flexible circuits. U.S. Pat. Nos. 4,987,100; 5,227,008; 5,334,487; 5,557,844 and 5,680,701 disclose processes for fabricating printed circuits having a flexible polymeric base substrate such as polyimide or polyester.
In some applications, flexible circuits may be exposed to an aggressive environment that promotes corrosion of the conductive traces. The conductive traces and the interface between the conductive traces and the base substrate are two areas susceptible to being adversely affected by environmental conditions such as exposure to corrosive fluids and moisture. To minimize the potential for corrosion, the conductive traces may include a corrosion resistant coverplate layer formed on a core or base layer of the conductive traces. It is common for the base layer to be made of a material susceptible to corrosion such as copper and for the coverplate layer to be made of a material resistant to corrosion such as gold, tin or palladium.
Gold is a preferred material for the coverplate layer in many applications. A coverplate layer made of gold exhibits excellent electrical conductivity, corrosion resistance and bonding performance. However, a gold coverplate layer is an expensive component of a flexible circuit. The cost associated with the coverplate layer can become excessive when the entire surface or nearly the entire surface of the base layer of each trace is plated with a coverplate layer made of a material such as gold, tin or palladium.
Some flexible circuits have a coverplate layer on only a portion of each trace to reduce the quantity, and therefore the cost, of the coverplate material. In most applications, the coverplated portions of the traces are functional areas used for bonding or testing of the circuit, or areas that need superior environmental protection in the end use environment. The portion of each trace that is not coverplated is often covered with a protective layer, such as a photoimageable or screen printed covercoat or adhesive film, to reduce the potential for corrosion of the base layer of the traces.
It is known in the art to use a selectively patterned protective layer as a photomask for the coverplate material. Subsequent to forming the patterned protective layer, the coverplate layer is formed on the base layer of the traces using a method such as electroplating or electroless plating. In both methods, the coverplate layer is formed on the base layer of the conductive traces, but not on the protective layer. After the coverplate layer is formed, the protective layer is left in place. This technique is more cost effective than applying a corrosion resistant coverplate layer to the entire circuit. However, this circuit fabrication technique results in the presence of an abutting interface between the protective layer and the coverplate layer. The inherent flexibility of flexible circuits, shrinkage of the protective layer and physical changes associated with thermal cycling of the protective layer contribute to reduced reliability and increased susceptibility to corrosion at the abutting interface.
Therefore, what is needed is a circuit construction and manufacturing process that reduces the potential for corrosion at the interface between the base layer and coverplate layer of a trace without requiring the entire base layer to be plated with a corrosion resistant coverplate layer.